In the process of investigating the problem of adriamycin cardiac toxicity, it became necessary to ask whether normal tissues differed in their complement of enzymatic defenses against damage by reactive oxygen species. This led us to compare cardiac tissue with liver and to discover that cardiac tissue lacked catalase. Subsequently, we have been able to enlarge upon this study and show further that cardiac mitochondria lacked matrix glutathione peroxidase while other tissues such as liver has this enzyme in abundance at that site. In the process of performing these studies, we discovered a new class of membrane bound glutathione peroxidase and are now actively trying to purify and characterize these enzymes.